LTC3859_15 Datasheet, PDF (27/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
APPLICATIONS INFORMATION
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV_CC) at maximum V_IN.
When the voltage applied to EXTV_CC rises above 4.7V, the
V_BIAS LDO is turned off and the EXTV_CC LDO is enabled.
The EXTV_CC LDO remains on as long as the voltage applied
to EXTV_CC remains above 4.5V. The EXTV_CC LDO attempts
to regulate the INTV_CC voltage to 5.4V, so while EXTV_CC
is less than 5.4V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 5.4V, up to an absolute maximum of 14V,
INTV_CC is regulated to 5.4V.
Using the EXTV_CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3859’s
switching regulator outputs (4.7V ≤ V_OUT ≤ 14V) dur-
ing normal operation and from the V_BIAS LDO when the
output is out of regulation (e.g., startup, short-circuit). If
more current is required through the EXTV_CC LDO than
is specified, an external Schottky diode can be added
between the EXTV_CC and INTV_CC pins. In this case, do
not apply more than 6V to the EXTV_CC pin and make sure
than EXTV_CC ≤ V_BIAS.
Significant efficiency and thermal gains can be realized
by powering INTV_CC from the buck output, since the V_IN
current resulting from the driver and control currents will
be scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTV_CC pin directly to V_OUT. Tying the EXTV_CC pin to
a 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T_J = 70°C + (40mA)(8.5V)(34°C/W) = 82°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV_CC power from
the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC left open (or grounded). This will cause INTV_CC
to be powered from the internal 5.4V regulator result-
ing in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV_CC connected directly to the output voltage of one
of the buck regulators. This is the normal connection
for a 5V to 14V regulator and provides the highest ef-
ficiency.
3. EXTV_CC connected to an external supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTV_CC providing it is compatible with the
MOSFET gate drive requirements. Ensure that EXTV_CC
< V_IN.
4. EXTV_CC connected to an output-derived boost network
off one of the buck regulators. For 3.3V and other low
voltage buck regulators, efficiency gains can still be
realized by connecting EXTV_CC to an output-derived
voltage that has been boosted to greater than 4.7V. This
can be done with the capacitive charge pump shown in
Figure 9. Ensure that EXTV_CC < V_IN.
LTC3859
V_IN1,2
C1
BAT85
BAT85
MTOP
TG
EXTV_CC
SW
MBOT
BG
L
R_SENSE
BAT85
V_OUT1,2
PGND
3859 F09
Figure 9. Capacitive Charge Pump for EXTV_CC
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors C_B connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C_B in the Functional Diagram is charged though
external diode D_B from INTV_CC when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the C_B voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns
on the topside switch. The switch node voltage, SW, rises
to V_IN for the buck channels (V_OUT for the boost channel)
and the BOOST pin follows. With the topside MOSFET
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